The present invention relates generally to deployable structures and more particularly to a spacecraft body having a deployable structure held in a stowed state by a holding device which is responsive to an environmental circumstance and is operative to release the deployable structure after the holding device is exposed to the environmental circumstance for a preselected criterion.
Deployable structures such as solar arrays and antennas are frequently used in spacecraft applications where a compact structure is required during a first stage and is deployed to a larger size in a second stage. Typically the first stage occurs when the spacecraft is coupled to a launch vehicle during the launch stage where the spacecraft is confined to a compact space. The second stage occurs when the spacecraft decouples with the launch vehicle in the deployment stage and subsequently expands in size by deploying the spacecraft' deployable structures such as antennas, booms, sensors, solar panels, and the like.
During the launch stage, deployable structures are positioned in a stowed state and secured in that stowed state with a holding device which has sufficient holding force to prevent the deployable structure from deploying until the desired time. At the desired time, an electrical signal is typically sent to the holding device instructing the holding device to release the deployable structure.
Clamps, bolts and gimbals are typically used as the holding device where the clamp opens, the bolt explodes or the gimbal moves in response to the electrical signal. Typically, the electrical signal used to release the holding device is generated by a human located in a ground control unit and is sent to the spacecraft. The drawback in this type of a system is that it is dependent on the spacecraft having the ability to receive and correctly process the electrical signal. If the spacecraft is oriented such that the electrical signal cannot be received, or, if the receive system is not functioning properly, the spacecraft will not receive the electrical signal and will subsequently not release the deployable structure which can be fatal to the spacecraft mission. In the case of deployable solar array, the solar arrays cannot deploy and the spacecraft will not be able to recharge the batteries resulting in a dead spacecraft. In the case of an antenna, the antenna cannot deploy limiting the communications capability of the spacecraft.
One method known to address this drawback is to provide an on-board spacecraft processor which is programmed to generate the electronic signal at a preselected time and send that signal to the holding device. The holding device releases the deployable structure in response to the signal thereby negating the need for ground control. However, this type of system is dependent on the processor to generate the electronic signal at the appropriate time which is in turn dependent on the reliability of the components within the processor. The components or processor can become damaged or fail such that the electronic signal is not generated.
What is needed therefore is a holding device which would release at a preselected time free of dependence on a human generated or electronically generated signal.